Remarkable Influence of Phenyl/Arylethynylation on the Photochromism of 2,2‐Diphenylbenzopyrans (Chromenes)

Phenyl/arylethynylation of 2,2‐diphenylbenzopyrans (i.e., chromenes) permits modulation of the spectral properties of photogenerated colored o ‐quinonoid intermediates, and brings about remarkable stabilization of the latter, rendering the phenomenon of photochromism observable readily at room temperature. Modification of the spectrokinetic properties of the o ‐quinonoid intermediates are a consequence of resonance effects, with the substituent effects being relayed nicely from one end of the molecule to the other. Functionalization at the 6‐position of the chromene nucleus leads to more redshifted absorptions as well as to increased lifetimes of the colored species when compared with functionalization at the 7‐position. Given that photochromism is observable in the parent chromene at only low temperatures, phenyl/arylethynylation, which precludes steric inhibition of resonance, is demonstrated to render bleaching kinetics of the colored intermediates that approach those reported for industrially relevant diarylnaphthopyrans.